1. Field of the Invention
The present invention relates to the use of tungsten and/or tantalum or compositions thereof, for inhibiting the accumulation of carbon on metal surfaces subjected to environments in which the decomposition of carbon-containing gases occurs.
2. Discussion of the Prior Art
Metal surfaces, especially those containing iron, nickel, chromium, cobalt, molybdenum, and alloys and combinations thereof, are prone to the accumulation of both filamentous and amorphous carbon when subjected to high temperature reactions involving carbon-containing materials, e.g., hydrocarbons and carbon monoxide. Examples of such reactions, which are of commercial importance, are the production of ethylene by cracking, the production of motor fuels from petroleum sources by conversion of heavy feedstocks, the production of vinyl chloride from dichloroethane and the production of CO and H.sub.2 by steam-reforming of hydrocarbon feed stock over a nickel-supported catalyst. Such reactions are generally accompanied by the accumulation of carbon on the surfaces of the reaction tubes in contact with the reaction medium. This accumulation of carbon in the reaction tubes causes a restricted flow of the reaction material and reduced heat transfer from the reaction tube to the reaction medium. It also causes damage to the inner surface of the tube owing to carburization and frequent exposure to the carburization/oxidation cycle also accelerates corrosion, both of which reduce reactor life expectancy. The reduction in heat transfer necessitates raising the reaction tube temperature to maintain a constant gas temperature and production rate.
Various methods have been employed to inhibit the accumulation of carbon. Such conventional methods include steam pre-treatment of the metal reactor inner-surface to promote formation of a protective oxide film. Also, sulfur compounds are added to the process gases to poison active nickel sites and to scavenge free radical precusors of amorphous carbon. However, the rate of carbon accumulation can still be rapid under high severity conditions.
Other methods taught in the prior art include the process, taught in U.S. Pat. No. 4,099,990, for forming protection films on nickel, chromium or iron alloy substrates susceptible to coke formation. The process consists of first preoxidizing the substrate surface, then depositing thereon a layer of silica by thermally decomposing an alkoxysilane vapor.
Another method is that taught in U.K. Pat. No. 1,529,441 wherein protective films are formed on a substrate of an iron, nickel or chromium, or alloy thereof. The protective film is applied by first depositing on the substrate surface a layer of another metal such as aluminum, iron, chromium or molybdenum by vaporization and then rendering this deposited layer insert by treatment with steam or a silicon compound.
Heat-exchangers in nuclear reactors can be protected against carbon deposits by use of certain volatile silicon compounds such as dichlorodiethylsilane. See U.S. Pat. No. 3,560,336.
Although many of these conventional methods have met with varying degrees of commercial success, there is still a need in the art for developing methods for protecting against the accumulation of carbon without adversely affecting the metal substrate. For example, although silicon compounds have proved commercially successful for protecting certain metal surfaces against the accumulation of carbon, there is still the possibility of an excess amount of silicon adversely affecting the properties of the metal substrate.